1. Field of the Invention
The present invention relates to multi-mode surface acoustic wave filter devices.
2. Description of the Related Art
Multi-mode surface acoustic wave (SAW) filters are well known as bandpass filters (see Proc. IEEE Ultrason. Symp., pp. 95–104, 1992, for example). A multi-mode SAW filter normally has input IDTs (interdigital transducers) and output IDTs arranged alternately, with SAW reflectors being provided at either end. The most common type of multi-mode SAW filter has one input IDT denoted by reference numeral 20 in FIG. 1, and two output IDTs denoted by reference numerals 30 and 40. A filter having this structure utilizes two resonance modes, and therefore, is often called a “double-mode SAW (DMS) filter”. A grating reflector 50 is provided in the propagation direction of surface acoustic waves on the external side of the output IDT 30. Likewise, a grating reflector 60 is provided in the propagation direction of surface acoustic waves on the external side of the output IDT 40. The input IDT 20 and the output IDTs 30 and 40 are formed on a piezoelectric substrate 10.
Each of the input IDT 20 and the output IDTs 30 and 40 has a signal terminal (input/output) and a ground terminal that are connected to a signal bus bar and a ground bus bar, respectively. A “bus bar” is a conductive thin film that is connected to IDT electrode fingers and extends in the propagation direction of surface acoustic waves. More specifically, the input IDT 20 has an input terminal 25 and a ground terminal 26 that are connected to a signal bus bar 21 and a ground bus bar 22, respectively. The output IDT 30 has an output terminal 35 and a ground terminal 36 that are connected to a signal bus bar 31 and a ground bus bar 32, respectively. Likewise, the output IDT 40 has the output terminal 35 (shared with the output IDT 30) and the ground terminal 36 (also shared with the output IDT 30) that are connected to a signal bus bar 41 and a ground bus bar 42, respectively.
In each of the input IDT 20 and the output IDTs 30 and 40, the signal electrode fingers extend from the corresponding signal bus bar to the corresponding ground bus bar. More specifically, the signal electrode fingers 23 of the input IDT 20 extend from the signal bus bar 21 to the ground bus bar 22. The signal electrode fingers 33 of the output IDT 30 extend from the signal bus bar 31 to the ground bus bar 32. Likewise, the signal electrode fingers 43 of the output IDT 40 extend from the signal bus bar 41 to the ground bus bar 42. Also, in each of the input IDT 20 and the output IDTs 30 and 40, the ground electrode fingers extend from the corresponding ground bus bar to the corresponding signal bus bar. More specifically, the ground electrode fingers 24 of the input IDT 20 extend from the ground bus bar 22 to the signal bus bar 21. The ground electrode fingers 34 of the output IDT 30 extend from the ground bus bar 32 to the signal bus bar 31. Likewise, the ground electrode fingers 44 of the output IDT 40 extend from the ground bus bar 42 to the signal bus bar 41.
In a conventional DMS filter, the input terminal and the output terminal are normally arranged opposite to each other, so that signals can be prevented from reaching the output terminal from the input terminal through the air or the substrate. Accordingly, the ground electrode fingers 24 of the input IDT 20 extend in the opposite direction to (180-degree different from) the ground electrode fingers 34 and 44.
FIG. 2 shows typical bandpass characteristics of a conventional DMS filter. In the 2 GHz band, an insertion loss of approximately −4 dB and a stop-band suppression of −35 dB are obtained.
The bandpass characteristics of a conventional DMS filter, however, exhibit poor steepness in the rising region and the declining region, as shown in FIG. 2. Also, there is a lump-like shoulder formed in the stop-band area on the high-frequency side, as shown in FIG. 2. Furthermore, the insertion loss becomes too great. Filters to be employed in communication systems for mobile telephone communications or the likes are normally expected to have only a small loss and to be able to exhibit a very steep decline at both ends of the pass-band area. Most conventional DMS filters have failed to maintain those characteristics that are expected in filters for mobile communication systems.